Add Change record to @deprecated for ResourcePluginManager

I personally think creating change records for trivial things like this is a waste of our time, but here it is: https://www.drupal.org/node/2874934.

The goal of these issues is not to add change record nodes, but to update the codebase documentation to link (hopefully existing) change records that explain how to convert from old APIs to new ones. So this issue is not completed yet, but we should figure out if the CR is correct or if a bigger, wider scope one that is more useful already exists.

#8: That was not clear to me when I read this in #5/#6, but now it is. Thanks for reopening!

Linked change record for @deprecated. Please review the patch.

Thanks!

Thanks @Wim Leers and @sorabh.v6. I updated the change record slightly to make the wording a little more neutral:
https://www.drupal.org/node/2874934/revisions/view/10465936/10527658

There is a slight coding standards error in the patch; there should be a blank line above the @see section. Checking for this standard will be added in #2842949: Fix Drupal.Commenting.DocComment.SpacingBeforeTags coding standard. Meanwhile, I fixed it on commit:

@@ -40,6 +40,7 @@ public function __construct(\Traversable $namespaces, CacheBackendInterface $cac
    * @deprecated in Drupal 8.2.0.
    *   Use Drupal\rest\Plugin\Type\ResourcePluginManager::createInstance()
    *   instead.
+   *
    * @see https://www.drupal.org/node/2874934
    */
   public function getInstance(array $options){

Committed and pushed to 8.4.x and cherry-picked to 8.3.x. Thanks!

Removing parent issue per conversation with @xjm at Drupalcon Nashville Mentored Sprint prep. Her recommendation to do so was based on a few points that made sense to all of us in the discussion, namely:
- so many child issues makes this parent unwieldy
- search filters will allow people needing to refind closed children